you shouldn't have any problem dual boot ubuntu and backtract 5, just create a partition using the ubuntu live cd (gparted) then point the backtract install to the same partition (choose the manual install process) and set the mount point for the existing swap partition, backtract 5 will replace the ubuntu grub, don't worry about it, it also uses grub 2 and you will still be able to boot ubuntu from grub.
I have a triple boot with windows 7, ubuntu 11.04 and backtract 5 (KDE) on 12GB partition.


